Traditional optical systems are used in many different applications all around the world. As an example, in security applications, a surveillance camera is often used to watch and follow a particular target like a person walking in a public place. When doing so, the analyst, may it be an automated computer or a human security officer, needs as much information as possible about the target. This thus requires better optical systems, using larger detectors and having as many detecting areas called pixels as possible receiving light from the desired target. However, as much as it is desirable to get a high quantity of pixels on the desired target, it is also useful to look at the largest scene as possible to cover a large field of view (FOV). The optical system designer is therefore confronted with a trade-off between a small FOV with a large number of pixels per degree and a larger FOV with fewer pixels per degree.
U.S. Pat. No. 6,898,021 to Tang discloses a combination of a zoom lens and a sensor for changing in real time the magnification using the zoom lens, and thus changing the FOV of the optical system. Increasing or reducing the magnification produces respectively a reduction or an increase of the FOV due to the optical invariant. The drawback is that by increasing the magnification on the desired target, the FOV is reduced and creates a blind zone. The increased magnification occurs along the optical axis.
U.S. Pat. No. 6,977,777 to Wick discloses an optical system in which a zoom lens is produced by using at least two active optics to change the magnification of the whole system by a given value. This has the same drawback of reducing the FOV when the magnification is increased and creating blind zones.
U.S. Pat. No. 6,215,519 to Nayar et al. discloses a combination of two or more imaging systems, one having a large FOV and the other a Pan/Tilt/Zoom (PTZ) lens looking directly at the target with high magnification and small FOV, in order to reduce the blind zone. Unfortunately, several cameras and lenses are needed to the expense of high data to be recorded and complex network.
Digital zooming using a sensor with a very large number of pixels is also possible. The sensor image can be displayed on various displays to view both large and narrow FOVs simultaneously. In this case, the narrow FOV is produced by a digital zoom rather than an optical zoom. Unfortunately, this system requires large format digital images to be transmitted, which increases the quantity of recorded information and requires larger bandwidth.
To reduce the size of the required bandwidth, several systems have been suggested. Some of those systems are based, for example, on the human eye which has a large FOV but a high spatial resolution at the center of the FOV due to a high concentration of photoreceptor.
For example, U.S. Pat. No. 6,421,185 to Wick et al. discloses a foveated imaging optical system that may be provided with a spatial light modulator to apply a wavefront filter to a zone in the FOV and provide a high resolution image in this zone while keeping a low resolution over the rest of the image. All the pixels of the high resolution zone in the image are transmitted, as well as a limited number of pixels from the rest of the image. Consequently, even if a high resolution sensor is required, a limited number of pixels are transmitted, limiting the required bandwidth.
U.S. Pat. No. 6,865,028 to Moustier et al. discloses a system in which, instead of using a high number of pixels on the sensor to increase the quantity of captured details, the magnification in the zone in which more information is needed is increased. Higher magnification means a high number of resolving elements or a high number of pixels are used in the zone of interest. In this system, a lens with a large FOV, with an embedded narrow FOV with a higher magnification, is used. The zone of increased magnification is obtained by a particular lens solution called Panomorph. The zone of increased magnification will use a large part of the sensor and then a large number of resolving elements or pixels. Therefore, the Panomorph solution produces a large FOV image with a higher magnification in the zone of interest. Unfortunately, the Panomorph lens is designed to provide a specific zone of interest as required by particular applications, and the zone of increased magnification is therefore fixed within the FOV. This prevents the position of the zone of increased magnification to be moved on the image.
There is therefore a need for an imaging system which would overcome at least one of the above-identified drawbacks.